1. Field of the Invention
This invention relates to a method and apparatus for forming and imaging press plates of the type which are mounted to the cylinders of a printing press and more specifically to a method and apparatus for accurately aligning the exposed image on the press plate relative to the inside leading edge bend of the plate in such a manner that the image will be properly aligned relative to the rotational axis of a specific press cylinder to which the plate will be mounted. In the present invention the press plate is initially formed having a forward or leading bent edge and is provided with punched holes along the opposite or trailing edge. The bent forward edge is formed so as to be cooperatively seated in the front end plate receiving slot of a press cylinder. The preformed plate is placed on a platen of an exposure machine and placed under proper tension and secured by a vacuum to the platen. A film to be exposed to the plate is placed over the plate and is thereafter aligned relative to the forward leading bent edge of the press plate by a pair of relatively adjustable reference pins. The purpose of the pins is to insure that the image to be exposed on the plate is angularly with the inside leading edge bend aligned to compensate for any deviation which may exist between the proper orientation of the front end plate receiving slot of a press cylinder and the rotational axis of the cylinder.
2. History of the Related Art
In printing presses such as commercial web presses, each printing cylinder is structured so that press plates may be selectively mounted to the cylinders. The press plates carry the image which is to be printed The cylinders are generally provided with at least one first slot for receiving the bent front edge of a press plate and at least one second slot which receives the trailing edge of the same press plate. Generally, a tensioning roll or other device is provided in the second slot so that the press plates are properly tensioned against the surface of the printing cylinders.
In order to insure proper alignment of the image carried by a press plate with respect to the rotational axis of a printing cylinder, the slot formed in each cylinder for receipt of the leading bent edge of the press plate is milled so as to be parallel with respect to the rotational axis of the cylinder. The current state of the art assumes that in the manufacturing process the slots in all of the printing cylinders of a given press are machined parallel to the rotational axis of each cylinder and that all of the cylinders of a press are properly and precisely aligned with respect to one another and remain so during the operation of the press. This, however, is not always the case.
On large presses with multiple plates across the printing cylinder and multiple plates around the cylinder, the positioning of individual press plates becomes very precise. It is inparitive that the plates which are individually mounted on one printing cylinder align with the plates on second and third cylinders respectively. For example, in the case where a four color layout is anticipated, there could be a total of 16 plates which on four cylinders each of which has to be mounted in absolute register with the other plates or the resultant printed image will not be satisfactory.
To further insure that the image on the printing cylinder is properly oriented with respect to the rotational axis of a cylinder, extreme care must be taken in preparing the press plates which are mounted to the printing cylinders. Conventionally, these plates are formed by aligning the plate in a punch device wherein holes are punched along one edge of the plate for purposes of receiving alignment or register pins. A stripping sheet with the imposed film having corresponding holes is then placed in position over the plate and a vacuum applied so as to bring the film into contact with the plate. Thereafter the plate is exposed by ultraviolet light. Once the plate has been exposed and treated, the plate is prepared for mounting to a printing cylinder. The plate is placed into a bending machine where the leading edge of the plate is bent so as to conform to the front end receiving slot of a printing cylinder.
From the foregoing procedure, it is necessary that each of the pin registers, the punched holes in the plates and in the strip film all be accurate so as to insure proper image alignment on the printing cylinder. In addition, the bending of the press plate must be accurate. Therefore, several different alignment operations must be manually completed without misaligning the images with respect to the plates. The tolerance in alignment of the image with respect to the inside leading edge bend of the plate must be in the range of approximately 1/1,000 of an inch in order to provide acceptable printing.
Due to the number of aligning operations and the bending of the press plate after it has been exposed, the current procedures for preparing the press plates often lead to possible alignment errors of the final image with respect to the press plate. To compensate for this, many commercial presses install cylinder cocking devices which allow for correction of any angular deviation of the plate image in relation to the leading or front edge slot formed in the printing cylinder. The cocking devices adjust the positioning of the printing cylinder itself as opposed to adjusting the positioning of the plate with respect to the cylinder. Unfortunately, many presses such as open web or non-heat set presses are not equipped with cylinder cocking devices and therefore if the image on a plate is out of alignment, the total plate must be re-made or the plate would have to be shimmed. By shimming, a plate is not properly seated with respect to a cylinder. When a plate is not seated properly with respect to the cylinders, the plates may work loose resulting in a position change of the image or a breaking or failure of the plate.
In presses which do incorporate cocking devices, the use of such devices places excessive wear on bearings, cylinder journals, gears and the like. In a printing press, the elements associated with the printing cylinders are manufacturing with tolerances up to the order of a few 0.0001 inch. Therefore, any wear on the bearings or journals can result in an out of alignment image. Further, the pressure exerted on the cylinders by the cocking devices changes the transfer pressure along the nip between the plate and the blanket cylinders which are engaged with the surface of the plates as the printing cylinders are rotated. This can cause deterioration in print quality.
Some of examples of prior art methods and apparatus for preparing printing plates are disclosed in U.S. Pat. No. 3,288,063 to Bungay, Jr., U.S. Pat. No. 3,771,440 to Eburn, Jr. and U.S. Pat. No. 4,529,303 to Ternes.